1. Field of the Invention
The present invention relates to electronic units for monitoring the useful life of devices designed for limited use such as water filters, for example, and more particularly to such units which provide signals informing the user of the current status of the device.
2. Description of the Prior Art
Many devices, such as disposable household water filtration cartridges, are designed for a limited useful life based on a number of discrete uses or elapsed time of use and perhaps additional factors such as flow rate, output water quantity, etc. Such cartridges are generally designed to filter a given quantity of water e.g., a certain number of fills of a specified container. However, there is no practical way for a consumer to manually keep track of the total number of container fills to determine when the filter cartridge is used up so that it may be replaced with a new cartridge. As a result most consumers will continue to use the filter until the water quality has degraded to the point of offending his or her sense of taste or smell. By that time the filter apparatus itself, rather than the used up cartridge, may be blamed for the poor water quality.
There have been several attempts to solve the problem of water filter cartridge overuse. U.S. Pat. No. 4,431,533 discloses a water filter in which water flowing through the cartridge completes an electric circuit between a battery and a visual or audio annunciator. Battery lifetime is selected so that the battery will run down and ceases to energize the annunciator when the filter cartridge is used up. However, the life of a battery will vary with factors in addition to the operating time of the filter such as temperature, non-operating time, etc. In addition, the indicator of the '533 patent does not inform the consumer when the end of the useful life of the cartridge is imminent so that a replacement cartridge can be obtained if not already on hand. Present devices require user participation and are prone to neglect if not used correctly.
U.S. Pat. No. 4,623,451 describes a home sink mounted water purification system which incorporates a purity sensing circuit including a pair of electrodes inserted in the filtered water for measuring current flow and/or a flow meter coupled to a timer for indicating the total quantity of filtered water. Visual indicators inform the operator that the water has acceptable levels of purity or that the apparatus should be serviced as a result of (a) the water purity falling below a preset level or (b) a predetermined quantity of water having been dispensed.
The '451 circuit dependent upon the incoming water quality is somewhat complex, does not count the number of uses of the cartridge and like the '533 device does not signal the consumer that the end of the cartridge life is imminent.
U.S. Pat. No. 4,683,054 discloses a tabletop water purifier which measures the time that the filter element is operational and also measures the efficiency of the filter element in removing dissolved solids. When the operational time reaches a predetermined amount or when the filter element efficiency drops below a preset value the operator is informed via a light signal of a potential requirement for change of the element i.e., end of its useful life.
U.S. Pat. No. 4,681,677 discloses a water filter apparatus which utilizes water pressure sensors to measure the pressure drop across a filter element and to activate a water shutoff valve upon reaching a preselected differential pressure.
U.S. Pat. No. 4,849,098 discloses an automatic water quality monitor for installation between a water filter and a conventional drinking water faucet. The monitor measures the total dissolved solids ("TDS") in the filter output and intermittently energizes a green light emitting diode ("LED") as long as the TDS is below a given value and energizes a red LED if that level is exceeded.
In addition to water filter elements, there are many other devices designed for use until a predetermined event has occurred such as (1) the last use of a given number of uses or (2) the expiration of a predetermined period of time or (3) the passage of a preset volume of material etc. There is a need for a small compact monitoring unit which may be fitted to such devices to inform the consumer or operator of the status of the device and particularly of the imminent end and the end of the useful life thereof.